With significant developments in micromachining technology, micro-electro-mechanical systems (MEMS) and nano-opto-mechanical systems (NOMS) have shown immense potential for sensor applications, which have advantages such as small size, low power packages and relative inexpensiveness. For example, optical MEMS pressure sensors based on Fabry-Perot interferometry have been reported. Compared to other MEMS sensing techniques such as piezoelectric, piezoresistive or capacitive, optical micromachined sensing devices operate by monitoring light properties, such as intensity or wavelength spectrum.
Optical sensors provide distinct advantages over capacitive-type and piezoresistive-type sensors, including: high sensitivity, immunity to electromagnetic interference (EMI), less read-out electronic complexity, low power consumption, easy telemetry applications, resistance to harsh environments, and capability for multiplexing.
There are two commonly employed sensing schemes for optical sensors, the first is by measuring the output intensity change at a certain wavelength and the second is by monitoring the resonance wavelength shift.
In Mach-Zehnder interferometer (MZI) based pressure sensors, due to applied pressure, phase change on the sensing arm of the MZI can be obtained as intensity changes.
In Fabry-Perot interferometer based pressure sensors, variations in cavity length due to the displacement of the diaphragm when pressure is applied can be obtained in terms of intensity variations. However, intensity interrogation typically has limited sensitivity. High detection sensitivity can be obtained by measuring intensity change at a fixed wavelength at the resonance peak of a high-Q resonator, but the light source needs to have a very accurate wavelength with a narrow bandwidth and high stability, which is difficult to achieve in practice. On the other hand, sensing schemes based on wavelength interrogation, i.e., measurement of the spectral shift of the resonance wavelengths, can be used with a high sensitivity.
Micro-ring resonators (especially Si micro-rings) have found numerous applications, which offer high quality factor (Q) and a compact size making such structures attractive for telecommunications and sensing applications. Micro-ring resonator based sensors use a wavelength-shift scheme, which is very useful for simultaneously reducing noise and enhancing sensitivity. Optical sensors are particularly viable for silicon photonics since crystalline silicon has superior optical properties, including high refractive index and low optical loss, which are not attainable with plastic materials. For Si waveguide-based sensors, the evanescent optical field expanded outside the Si waveguide can sense the surrounding variations. Moreover, devices having a ring-resonator configuration can further amplify the sensing response as light circulating inside the ring effectively and multiply interacts with the surroundings. Furthermore, high quality factor (Q) ring resonators have a longer effective interaction length with the surroundings, leading to an increase in sensitivity.
A need therefore exists to provide micro-machined optical pressure sensors that seek to address at least the above-mentioned problems.